The esterification of citric acid with varying molar ratios of long chain fatty alcohols can yield the mono-, di-, or tri- ester depending upon the molar ratio of the reactants. For example, EPO Published Application 0 199 131 reveals that citric acid may be esterified with one, two or three mole equivalent of ethoxylated long chain alcohols to yield products which consist mainly of mono-, di-, or tri- esters. The reaction of one mole of citric acid with about 2 moles of a long chain fatty alcohol yields mixtures of mono-, di-, and tri- ester compounds (U.S. Pat. No. 2,518,678). It is difficult to isolate the desired pure mono- or di- esters from these mixtures because of their similar solubility characteristics.
It is generally agreed that the predominant mono-ester product produced from the esterification of citric acid is the unsymmetrical 1-mono ester, while the predominant di-ester obtained is the 1,3 symmetrical product. Procedures have not been reported which reveal how to obtain the kinetically unfavored 2- mono substituted esters. Such a method has now been found. Likewise, there has been discovered a method to regioselectively prepare 1- mono substituted citric acid esters, free of other isomers. It is advantageous to have a method which can selectively prepare quantities of 1- or 2- citric acid mono ester compounds in the pure state so that their physical-chemical attributes can be assessed. Selectively substituted citric acid esters can also serve as building blocks for other synthetic studies.
The present invention involves the regioselective preparation of 1- and 2- mono substituted citric acid esters and the use of such esters as surface active agents.
Claims have been made for the use of partially esterified citric acid derivatives as:
Surfactants: Japan Patent 64-22336 (1989) Citric acid di-ester mono-salt as an anionic surfactant for detergents with reduced foaming and good penetration properties. EPO: 86104119.2 (1986) Surfactants derived from citric acid and aliphatic polyoxyalkylated alcohols: Borchert and Hartford, Proceedings World Surfactant Congress. "Surfactants in Our World - Today and Tomorrow". 1984, Munich, West Germany, Volume II. p. 147. Compositions consisting mainly of citric acid di-esters as surfactants. CA 92:997002.
It is generally recognized that the uncatalyzed or acid catalyzed reaction of citric acid with excess molar quantities of alcohols yields primarily the unsymmetrical 1- substituted compounds as the kinetic product. Lee and Miller report in J. Org. Chem., 1983, 48, 24 that the product of the reaction of anhydromethylene citric acid, as the di-triethylammonium salt, with short chain alcohols, i.e. C-1 to C-3 carbon atom alkyl alcohols and benzyl alcohol, in refluxing chloroform, yields only the symmetrical 2-mono ester. Anhydromethylene citric acid is prepared by the fusion of citric acid and paraformaldehyde.